In the visualization of the vascular vessel tree, the blood flow, the plaque formation, the plaque structure and further CFD parameters the following standard methods are used:
1. Imaging with 3-D Image Modalities
The 3-D image modalities serve to enable reconstruction of the vascular vessel tree. To this end a contrast medium is injected, in order to strengthen the blood vessels to be observed. The segmentation of the vessels from a 3-D data set represents the vascular lumen, which is used as the input for CFD methods.
2. Intravascular Imaging (IVB)
The intravascular imaging, for example IVUS (Intravascular Ultrasound) or OCT (Optical Coherence Tomography), enables a mapping and analysis of the vessel wall, for example for the representation of plaque. The virtual histology function of IVUS provides automatic means for the detection and classification of plaque, for example the fat content, the fibrousness and the calcification.
3. CFD Method
The simulation of the blood flow by means of the CFD method (Computational Fluid Dynamics) delivers a three-dimensional distribution of the flow parameters, such as for example WSS (Wall Shear Stress), along the surface of the vessel lumen.
3-D image modalities
3-D DSA rotation angiography is a standard method used for the assessment of vascular anatomy before and during interventions. In digital subtraction angiography (DSA), after the creation of mask images, images without contrast medium, and filling images, images with contrast medium, these are subtracted from each other, so that only the temporal changes arising as a result of the contrast medium are obtained, which represent the vessels.
In neuroradiology especially, three-dimensional digital subtraction angiography (3-D DSA) is a routine tool for the planning and execution of minimally invasive procedures. Modem neurological operating theaters integrate this 3-D capability too with a rotating C-arm, in order to enable pre- and intra-procedural 3-D imaging of the cerebral blood vessels.
A C-arm x-ray machine for digital subtraction angiography represented by way of example in FIG. 1 has for example a C-arm 2 rotatably mounted on a stand in the form of a six-axis industrial or articulated-arm robot 1, on the ends of which are attached an x-ray emission source, for example a x-ray emitter 3 with x-ray tube and collimator, and an x-ray image detector 4 as an image acquisition unit.
By means of the articulated-arm robot 1 known, for example, from U.S. Pat. No. 7,500,784 B2, which preferably has six axes of rotation and thus six degrees of freedom, the C-arm 2 can be spatially adjusted at will, for example by being rotated about a center of rotation between the x-ray emitter 3 and the x-ray detector 4. The inventive x-ray system 1 to 4 is in particular rotatable about centers of rotation and axes of rotation in the C-arm plane of the x-ray image detector 4, preferably about the center point of the x-ray image detector 4 and about the axes of rotation bisecting the center point of the x-ray image detector 4.
The known articulated-arm robot 1 has a base frame, which for example is fixedly mounted on a base. Thereupon is fixed a carousel which can be rotated about a first axis of rotation. Arranged pivotably about a second axis of rotation on the carousel is a robot pinion, to which is attached a robot arm, which can be pivoted about a third axis of rotation. On the end of the robot aim is arranged a robot hand, which is rotatable about a fourth axis of rotation. The robot hand has a fixing element for the C-arm 2, which can be pivoted about a fifth axis of rotation and rotated about a sixth axis of rotation running perpendicular thereto.
The realization of the x-ray diagnostic device is not reliant on the industrial robot. Conventional C-arm devices can also be employed here.
The x-ray image detector 4 can be a rectangular or square, flat semiconductor detector, which is preferably created from amorphous silicon (a-Si). Integrating and possibly counting CMOS detectors can, however also be employed.
A patient 6 to be examined as the object under examination is located in the beam path of the x-ray emitter 3 on a patient couch 5, for scanning a heart, for example. Connected to the x-ray diagnostic apparatus is a system control unit 7 with an image system 8, which receives and processes the image signals from the x-ray image detector 4 (operating elements are, for example, not shown). The x-ray images can then be examined on a monitor 9.
Other systems, for example for neuroradiology, use two C-arms. These are so-called biplane systems, as described in greater detail with reference to FIG. 2.
These essentially have two so-called planes, wherein the first plane 10 can comprise the x-ray diagnostic device shown in FIG. 1 with C-arm 2, x-ray emitter 3 and x-ray image detector 4. Via a ceiling bracket 11 a ceiling-hung C-arm 2′ with an x-ray emitter 3′ and an x-ray image detector 4′ of a second plane 12 can be provided. A monitor traffic light 13 with a first display 14 for the first plane 10 and a second display 15 for the second plane 12 can likewise be arranged on the ceiling. A high-voltage generator 16 is provided alongside the system control unit 7.
Intravascular Imaging (IVB)
In order to render the plaque more readily visible, a separate IVUS (Intravascular Ultrasound) catheter can be introduced into the vascular vessel tree of a patient. Such an IVUS system is for example described in DE 198 27 460 A1, from which a method for intravascular ultrasound mapping is known, in which an ultrasound signal transmitter and detector is introduced into a body lumen, and within which this can be moved. The ultrasound signal transmitter and receiver transmits ultrasound signals and captures reflected ultrasound signals, which contain information about the body lumen. A processor coupled to the ultrasound signal transmitter and receiver derives a first image series and a second image series from the captured ultrasound signals, and compares the second image series with the first image series. The processor can also be programmed to monitor the first and second image for cardiovascular periodicity, image quality, temporal change and vessel movement. It can also assign the first image series and the second image series to each other.
For intravascular imaging (IVB), a generally known OCT-catheter (Optical Coherence Tomography) can however also be introduced into the vessel.
CFD method
In DE 10 2008 014 792 B3 a method for the simulation of a blood flow in a vessel section is described, wherein an image acquisition of a vessel area encompassing the vessel section is obtained, a 3-D vessel section model is determined from the image acquisition, a number of blood flow parameters are read in, taking account of the, or of each blood flow parameter, the blood flow in the vessel section model is simulated and a number of hemodynamic parameters are output. It is here provided for that the image acquisition with an implant introduced into the vessel section is obtained in such a way that image data of the implant is included, and that the 3-D vessel section model is determined taking account of the image data of the implant employed. Further, a corresponding device for the simulation of a blood flow in a vessel section is specified.
In today's medical world, the dynamic behavior of the blood flow in an aneurysm is frequently regarded as an important factor for the pathogenesis of the aneurysm, that is for its occurrence and development.
As is known from the article “Image-Based Computational Simulation of Flow Dynamics in a Giant Intracranial Aneurysm” by D. A. Steinman, J. S. Milner, C. J. Morley, S. P. Lowie and D. W. Holdsworth from the American Journal of Neuroradiology (2003), Number 24, pp 559 through 566, a number of so-called hemodynamic parameters are connected with a growth and a rupture of the aneurysm. A hemodynamic parameter is in particular understood to be a parameter relating to hemodynamics, that is the flow dynamics of the blood. As hemodynamic parameters the article cited includes a pressure, a shear stress affecting the vessel wall and a flow rate.
In order to draw conclusions about hemodynamic parameters of this kind, for example, the blood flow in a vessel section, which for example includes the aneurysm, is for example simulated.
To this end, in the aforementioned article “Image-Based Computational Simulation of Flow Dynamic in a Giant Intracranial Aneurysm” a 3-D vessel section model is determined from a 3-D image acquisition, which was obtained by means of a rotation angiography. The blood flow in the 3-D vessel section model is simulated by means of the CFD method. The simulation is here performed assuming rigid vessel walls and a constant blood viscosity. CFD is a method of numeric flow simulation. The model equations used in the numeric flow mechanics are mostly based on a Nervier-Stokes equation, on a Euler or potential equation.